The present invention relates to a power transmission for use in the driveline of an engine-powered industrial vehicle such as a forklift or a shovel loader and of the type equipped with fluid actuated clutches of which friction facings operate in hydraulic fluid.
A conventional power transmission of the above-mentioned type is equipped with a fluid control system, which is constructed to initially regulate fluid pressure produced by a pump to a predetermined value and thereafter supply the regulated pressure to a manual selector valve which in turn distributes the regulated pressure to one of the clutches in accordance with the shift position of the selector valve which is manually operated by a vehicle driver whereby a forward drive power train or a reverse drive power train is accomplished.
As is well known in the art, the power transmission mounted on the industrial vehicle is quite frequently subjected to such an operative condition that its clutches are operated in a semi-clutched condition. The semi-clutched condition will be used throughout this disclosure to indicate the operative condition of a clutch wherein its friction surfaces are held loosely in contact with each other for allowing the slippage or the relative movement therebetween whereby the clutch can transmit a small amount of the torque from the driving member to the driven member.
Such frequent slippage causes the friction facings to generate heat and to wear excessively. In order to prevent such excessive heat generation and wear, it is necessary to lubricate the friction facings with a hydraulic fluid.
Two types of clutches to be incorporated in the power transmission of an industrial vehicle are known in the art, one type of which has friction facings made of metal and the other type of which has friction facings made of impregnated paper. The clutch equipped with metal friction facings is superior to a clutch equipped with paper friction facings in that the amount of fluid necessary for lubricating the former clutch is less than that necessary for lubricating the latter clutch. Furthermore, the resistance to wear of a metal friction facing is far superior to that of a paper friction facing.
However, a metal friction facing is far more expensive than a paper friction facing thus making the cost of the clutch equipped with metal friction facings quite high.
In order to reduce the cost of the power transmission of an industrial vehicle, it is therefore desirable to employ a clutch equipped with paper friction facings instead of a clutch equipped with metal friction facings.
In case clutches equipped with paper friction facings are employed, however, it is necessary to construct the fluid control system of the power transmission such that it can supply the clutches with adequate amounts of lubrication fluid, which makes the fluid control system quite complicated as will be apparent from the following, thus increasing the cost thereof to the extent of counterbalancing the reduced cost which is accomplished by employing the clutch equipped with paper facings.
For the reasons discussed above, clutches equipped with metal friction facings have been employed for almost all conventional power transmissions for use in industrial vehicles.
Referring to FIG. 1, explanation of a conventional fluid control system for controlling a power transmission equipped with friction clutches having paper friction facings is made.
In FIG. 1, an engine 10 drives a hydraulic pump 12 which pumps hydraulic fluid from a fluid reservoir 14 through a screen 16 to supply fluid pressure to clutches 18 and 20 by way of a relief valve 22 in the form of a normally closed valve, a variable throttle 24 and a manual selector valve 26 in the form of a four-way-three position (4/3) valve. The selector valve 26 is operable to selectively distribute the fluid under pressure to one of the clutches 18 and 20 in accordance with its shift position selected by a vehicle driver whereby the vehicle is driven by engine 10 in the forward or reverse direction, or held to stall. Reference character 28 indicates a pressure regulator valve which is operable to regulate the pressure supplied to actuate the required clutch to a predetermined value, e.g., when the pressure exceeds the predetermined value the regulator valve opens its drain port thereby to exhaust the excess pressure. The pressure regulator valve 28 is further operable, in response to the movement of a piston 30 which is usually operatively connected to a brake pedal (not shown), to reduce or eliminate the clutch actuation pressure thereby to move the clutch from an engaged position to a semi-clutched or declutched position. When the clutch is moved as above by the operation of the piston 30, the relief valve 22 is closed in order to retain a proper supply of fluid from the pump 12 to a torque converter 32 so that the proper converter pressure is maintained. The torque converter 32 is supplied with fluid under pressure discharged from the pump 12 via a flow restriction 34 and partially exhausts its working fluid into a return flow passage 36 in which is arranged a relief valve 38 for maintaining the converter pressure at a predetermined value, for example at 3 kg/cm.sup.2. Reference numeral 40 indicates an emergency relief valve which is operable to drain an excessive portion of the fluid supplied to the torque converter when the supply of fluid becomes excessive thereby preventing breakage of the torque converter. The fluid exhausted from the converter into the return flow passage 36 is conducted via the relief valve 38 to the clutches 18 and 20 for lubrication of their friction facings (not shown).
A lubrication fluid control valve 42 in the form of three way-two-position (3/2) valve is pronded and which is operatively connected to the pressure regulator valve 28 and functions to selectively distribute the fluid drained from the pressure regulator valve to lubrication points of the transmission or the torque converter as follows in accordance with its actuated position. The position of the lubrication control valve 42 is controlled by the combined movement of a first pressure sensing device 44 which is movable in response to changes in the pressure supplied to the torque converter and a second pressure sensing device 46 which is movable in response to changes in the pressure supplied to the clutches. Since the pressure supplied to the torque converter is maintained approximately at a constant value in order to prevent the cavitation of the converter, the lubrication fluid control valve 42 is operable to change its valve position in response to the movement of the piston 30, i.e., when the piston is moved upwardly as viewed in the drawing, the valve 28 opens its drain port thereby to reduce the clutch actuation pressure thus causing the valve 42 to have the position, as depicted in the drawing, where it distributes the fluid drained from the valve 28 to the foregoing lubrication points of the power transmission for example to the idler shafts.
When, on the contrary, the clutch actuation pressure increases, the lubrication control valve 42 is actuated to move to assume a position which is reversed to the position as depicted in the drawing. In this position, the lubrication fluid control valve distributes the fluid drained from the pressure regulator valve 28 to the torque converter through a check valve 48 thus causing the amount of fluid exhausted from the converter to increase thus to increase the amount of lubrication fluid supplied through the relief valve 38 to the clutches for lubrication thereof.
A by-pass passage arranged with a flow restriction 50 provides communication at all times between the pressure regulator valve 28 and the aforementioned lubrication points of the transmission for supplying the fluid drained from the pressure regulator valve 28 to said points.
From the above explanations given to the conventional fluid control system for controlling a power transmission equipped with clutches having friction facings made of impregnated paper, it will be appreciated that the fluid control system of the foregoing type is complicated in construction mainly for the reason that the control system indispensably requires the provision of the relief valve 22 for preventing the converter pressure from falling below a proper value even when the valve 28 is moved by the piston 30 to have a position where it drains the fluid pressure supplied to actuate the appropriate clutch, and the lubrication fluid control valve 42 and the sensing devices 44 and 46 for moving the clutch to a semi-clutched or declutched position when the vehicle brake pedal is depressed moving the piston 30.
The conventional fluid control system of the foregoing type encounters a further drawback that it cannot increase the amount of lubrication fluid supplied to the clutch when the clutch is in a semi-clutched condition. When the clutch is in a semi-clutched condition, the lubrication fluid control valve 42 is in a position to supply the fluid drained from the pressure regulator valve 28 to the transmission lubrication points, for example the idler shafts. In order to prevent excessive heat generation and wear of the friction facings of the clutches, particularly in the case of the friction facings being of impregnated paper, it is quite useful and effective to construct the fluid control system so that it can increase the amount of lubrication fluid supplied to the clutches when they are in a semi-clutched condition.